1. Field of the Invention
The present invention relates to a transmission electron microscope and, more particularly, to a scanning transmission electron microscope using optical fibers grouped into fiber bundles so as to be adapted for optical detector segments of arbitrary shapes.
2. Description of Related Art
A transmission electron microscope (TEM) is an instrument permitting observation of a transmission electron microscope image (TEM image) or a diffraction image by irradiating a specimen with an electron beam emitted from an electron gun and forming such an image from electrons transmitted through the specimen.
Where a TEM image is obtained with a transmission electron microscope, a transmitted electron detector in an annular form is used. A scintillator is used to convert a transmitted electron image into an optical signal. An optical detector is used to convert the optical output from the scintillator into an electrical signal. Various kinds of optical transmission media are used from the scintillator to the optical detector.
FIG. 6 shows the configuration of a related art photoelectric converter. Transmitted electrons e impinging on one scintillator 1 are converted into an optical signal by the scintillator 1 and guided via a light guide 2 to a photomultiplier (PMT) 4 that is an optical detector. As an example, optical fibers are used as the light guide 2. Transmitted electrons e incident on another scintillator 1 are converted into a light signal by the scintillator 1 and guided to another photomultiplier (PMT) 4 via a light guide 3.
The configuration 1A, 2, 4A shown in FIG. 6 is known as a high-angle annular dark-field (HAADF) detector. The configuration 1B, 3, 4B shown in FIG. 6 is known as a bright-field (BF) detector. In actual instrumentation, the BF detector is disposed behind the HAADF detector. High-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) is one dark-field imaging method and is used to detect and image those transmitted electrons e which are scattered through large angles with an annular detector.
A known instrument of this kind is described, for example, in JP-A-8-285947 (paragraphs [0020-0028]; FIG. 6) and is an electron beam detection system for use with an electron microscope. The detection system is composed of a scintillator, a transparent substrate, optical lenses, an avalanche imaging device, an imaging device controller, a computer, and a monitor. The detection region of the sensitive surface of the imaging device is partially set by the imaging device controller.
Furthermore, a technique using a four-segmented radiation detector is known as described, for example, in JP-A-61-39442 (from page 2, right lower column, line 2 to page 3, right lower column, line 12; FIGS. 1 and 2). The radiation detector is matched to a scanning pattern. Individual signals are recorded for desired ones of the line segments of the scanning pattern. In addition, a structure having plural conductive beam-sensitive surfaces which are disposed in the path of a charged-particle beam and which are successively arranged radially and peripherally from the center is known (see, for example, JP-UM-A-63-99288).
Additionally, a medium which transmits an electron image to a detector within an electron microscope and which couples together the ends of a scintillator plate and bundles of optical fibers is known (see, for example, JP-A-7-281036 (paragraphs [0013-0018]; FIGS. 1-3)).
The related art scanning transmission electron detector has the problem that information about an angular distribution or directional distribution of scattered electrons cannot be precisely obtained because the scattered electrons are averaged with respect to angle or direction before being detected. In order to obtain angular information with conventional detectors, an HAADF detector and a BF detector which are different in annular size are used. In this case, the number of segments and their shapes are greatly limited. Furthermore, in the related art method using a light guide, it is impossible even to split a detector into four segments as shown in FIG. 7.
With the segmented TEM detector, it is necessary to control the positional relationship between a diffraction pattern and the detector segments. In the conventional method, the positional relationship must be controlled by the use of a magnetic lens or by rotating the detector itself. With these methods, it is quite difficult to accomplish a complete positional control. It is necessary to change the state from the state shown in FIG. 4A to the state shown in FIG. 4B. In the state FIG. 4A, there are shown a 4-segment detector 10 and a diffraction pattern 11. Under this condition, it is necessary to control the symmetry between the diffraction pattern and the detector as shown in FIG. 4B.